In the field of rubber industry, especially, automobile components are desired to have advanced function and performance. A frictional power transmission belt is one of rubber products used in such automobile components, and the frictional power transmission belt is widely used for, for example, power transmission of auxiliary drive of automobile air compressors, alternators and the like. Although a V-ribbed belt having ribs provided along a belt longitudinal direction has been known as the belt of this kind, sound generation resistance is required for the V-ribbed belt, in addition to the belt performance such as fuel-saving performance and abrasion resistance. Particularly, in running at the time when exposed to water, generation of stick-slip sound becomes a problem. Specifically, if wettability of the frictional power transmission surface is low and a water penetration condition between the belt and pulleys is uneven, a friction coefficient is high at a portion to which water does not penetrate (dry state) and a friction coefficient remarkably decreases partially at a portion to which water penetrates (water-exposed state), so that the friction state becomes unstable and the stick-slip sound is generated.
Patent Document 1 discloses a frictional power transmission belt in which at least a frictional power transmission surface is composed of a rubber composition in which 1 to 25 parts by mass of a surfactant is blended relative to 100 parts by mass of an ethylene-α-olefin elastomer. As for this frictional power transmission belt, affinity of the rubber (ethylene-α-olefin elastomer) forming the frictional power transmission surface to water can be enhanced by blending the surfactant and thus, abnormal noise caused by stick-slip can be reduced to improve the sound generation resistance when exposed to water.
However, in this belt, although the surfactant oozed out on the frictional power transmission surface stabilizes the friction state between the belt and the pulleys, probably because of unstable behavior of the surfactant in the rubber, internal loss (tan δ) increases and torque loss becomes large. In addition, rubber strength decreases and thus there is a concern that the abrasion resistance cannot be maintained. Furthermore, in the case where the surfactant is blended all over the compression rubber layer, there is also a concern of a decrease in mechanical characteristics (strength, elongation, etc.) of the compression rubber layer.
Patent Document 2 discloses a power transmission belt having a compression rubber layer provided at the belt bottom side of a tension member, in which short fibers composed of a gellable polyvinyl alcohol fiber subjected to an RFL treatment are embedded in the compression rubber layer so as to be exposed on the surface of the compression rubber layer. This document describes that, even when a large amount of water enters, a water film is broken through and thus a decrease in power transmission ability and generation of abnormal noise, which may be caused by slipping resulting from a water layer generated at an interface between the belt and the pulleys, are prevented by water absorption and gelation of the exposed polyvinyl alcohol short fibers. Furthermore, in Examples, the belt was rotated on a two-axle testing machine and sound generation limit tension during water pouring was measured as evaluation of sounding performance.
However, since the short fibers contained in this power transmission belt are gellable polyvinyl alcohol fibers, sound generation resistance cannot be improved. That is, in Examples of Patent Document 2, a load at 2% slipping during water pouring was measured but the load becomes large as compared with Comparative Examples (Nylon short fibers were blended) and the friction coefficient during water pouring increased. However, in an actual vehicle engine, because of rotational fluctuation, if the friction coefficient during water pouring is high, the sound generation by stick-slip is prone to occur. Therefore, it is necessary to enhance affinity of a rubber forming the frictional power transmission surface to water and to form a uniform water film so as to decrease the friction coefficient during water pouring and also to decrease a change in the friction coefficient to a slip rate. In the short fibers of Patent Document 2, however, since the water-absorbed and gelled short fibers protrudes on the frictional power transmission surface and breaks through and removes the water film, a uniform water film itself cannot be formed and thus a friction state cannot be stabilized. Therefore, in the actual vehicle engine having rotational fluctuation, the sound generation resistance is not sufficient.
Moreover, the gelled short fibers are presumed to be softened due to water absorption but, since the short fibers that have protruded are abraded at the time of belt power transmission, the abrasion resistance cannot be also maintained.
Furthermore, the short fibers are difficult to disperse into the compression rubber layer as compared to particles, and processability is low. In addition, since the short fibers dispersed in the rubber have small contact area with the rubber and have a smooth contact surface, adhesiveness to the rubber decreases and thus a surface treatment such as a resorcin-formalin-latex (RFL) treatment becomes necessary for improving adhesive force. In addition, since the short fibers are blended all over the compression rubber layer, there is a concern of a decrease in mechanical characteristics.
Patent Document 3 discloses a frictional power transmission belt in which at least a part of a frictional power transmission surface is composed of a rubber composition containing from 5 to 50 parts by mass of a water-soluble polymer having a melting point or a softening point of 80° C. or lower relative to 100 parts by mass of a rubber. In this document, as the water-soluble polymer, polyethylene oxide is described.
However, this water-soluble polymer melts at the time of vulcanization of the belt and hence, is dispersed in all over the compression rubber layer, but the internal loss (tan δ) increases and the torque loss becomes large probably because the molten water-soluble polymer inhibits crosslinking of the rubber. Moreover, since the water-soluble polymer is blended all over the compression rubber layer, there is a concern of a decrease in mechanical characteristics. Incidentally, in Examples of Patent Document 3, polyvinyl alcohol was blended as a water-soluble polymer but the case is described as Comparative Example in which sound generation limit tension during water pouring is low and also details thereof are unknown.
Patent Document 4 discloses a frictional power transmission belt comprising a compression rubber layer that comes into contact with pulleys to transmit power, in which the compression rubber layer has a surface rubber layer having a relatively large content of a plasticizer and containing a particulate ultrahigh molecular weight polyethylene resin and an inner rubber layer having a relatively small content of a plasticizer. In addition, Patent Document 5 discloses a frictional power transmission belt containing a compression rubber layer having a frictional power transmission surface for engaging with or coming into contact with pulleys, in which a lubricant formed of a polyethylene-based resin is attached to the frictional power transmission surface.
However, with the polyethylene-based resin such as the ultrahigh molecular weight polyethylene resin, the sound generation resistance and the abrasion resistance can be improved by reducing the friction coefficient but sound generation at the time when exposed to water cannot be suppressed at a high level.